Many original equipment manufacturers of products such as toys, typewriters, appliances, desks and other office equipment have established very demanding performance characteristics for the coatings used on their products. At the present time, these performance characteristics are capable of being met only with high temperature cure coatings, and both solvent-based and water-based compositions are currently available for this purpose. As these manufacturers broaden their choice of substrates from steel and aluminum into a variety of plastic materials, it becomes unacceptable to use high temperature cure coatings. This is because the plastics typically used, such as Noryl synthetic thermoplastic resins, Lexan polycarbonate resins, polystyrene, ABS and injection molding grade polyesters, are thermoplastic and tend to soften and deform at the temperatures of 300.degree. F. and higher that are required to cure the existing coating compositions. Moreover, most such coatings are solvent-based and, for environmental reasons, it is preferred to use aqueous-based compositions to avoid the pollution problems associated with solvent-based compositions.
Typical performance characteristics that must be possessed by a topcoat composition for many of these products include texturing capability, resistance to solvents such as 1,1,1-trichloroethane commonly used as an ink stain cleaner and adhesive remover, impact resistance, gloss level, hardness, dry adhesion and humidity resistance, as well as other known performance characteristics. The coating properties that would give satisfactory performance for solvent resistance (hardness) and impact resistance (flexibility) are generally the hardest to achieve simultaneously in a coating composition because they tend to be somewhat incompatible in nature. Additionally, satisfactory adhesion to a variety of metal and plastic substrates is a difficult problem to resolve when using aqueous compositions which are considered desirable to employ for environmental control reasons.
In copending application Ser. No. 06/459,417, filed Feb. 24, 1983, now abandoned entitled "Aqueous Coating Composition Comprising a Dispersion of Polymerized Unsaturated Monomers, a Nonionic Surfactant and Crosslinking Agent" and assigned to the assigned the assignee of the present invention, there is disclosed and claimed a topcoat and emulsion which is capable of exhibiting the desired performance characteristics and is also advantageous for use in association with thermoplastic substrates because of its low curing temperature, e.g. 150.degree. F. However, while it may in some cases be used as both the base coat and topcoat on certain substrates, such as Lexan polycarbonate and Noryl synthetic thermoplastic resins, it lacks proper adhesion on polyester substrates. Moreover, because it is strongly acid catalyzed, it is not appropriate for direct application to metal substrates on which corrosion would result from the presence of the acid catalyst.
It is therefore an object of the present invention to provide a low temperature cure coating system capable of exhibiting high performance characteristics and which is compatible with both metal and plastic substrates.
This and other objects of the invention will become apparent in the course of the following disclosure.